I wondered the same thing! But then again, I'm so new to all of this that I have questions about nearly every post. I'm trying to pace myself on how many things I ask about ... 
270 Offy
- Thread starter mayhugh1
- Start date
-
- Tags
- 270 offenhauser offy
Help Support Home Model Engine Machinist Forum:
KennyMcCormick315
Active Member
- Joined
- Jul 4, 2014
- Messages
- 29
- Reaction score
- 6
Model aircraft engines have a lot of oil coming in with the fuel. 15-20 percent of the total fuel volume, depending on blend. Mine suck down an 18% lube package, of that, half is castor oil and half is synthetic. Rest of the fuel is, for the most part, methanol with a whiff of nitromethane for that extra kick.
Same exact fuel my 2-cycles run on, in fact.
If you want to simplify lubrication...and ignition...on your BR2 build, aim for a CR of around 7.5:1 and use model aviation fuel in it. Glow plugs use a quarter-20 thread and the fuel is designed to work with them. Give 'em 1.5v on startup and, once it's warmed up, it should self-sustain from there.
Some of the larger ones run on gasoline, use a CDI ignition system, and use the same oil that your strimmer uses. Saito recommends a 16:1-20:1 mix in theirs.
Glow plugs are 1/4 32 thread
KennyMcCormick315
Active Member
- Joined
- Jul 4, 2014
- Messages
- 29
- Reaction score
- 6
Oh, they're fine thread? Huh. Well, either way, they're all standardized to a common thread. 5/16 hex.
I'd been procrastinating over a number of loose ends related to the Offy's oiling system, and so I decided to tie them up before the holidays. First, the crankcase passages behind the pumps were drilled using the holes' coordinates and a previously machined drilling template as a sanity check. Two of the risky small diameter holes were nearly five inches deep. After starting out with jobber length drills, I switched to aircraft drills that I had on hand, but their long flute-less shanks tended to heat up and bind inside the holes even at half depth. I eventually switched to using long Guhring drills, but in order to reach full depth I could only grip the upper eighth inch of their shanks. The drilling of these two passages was sketchy, but I managed to get through it without breaking off a drill deep inside a finished crankcase.
The oil manifold holes on the lower front side of the crankcase were trivial in comparison. A .010" thick Teflon gasket was made to seal the manifold to the crankcase.
The tortuous path between the scavenger pump's output and the engine's top-end was completed up through the top of the gear tower. A 2-56 grub screw located in the side of the engine will eventually control its flow. A 3.5mm o.d. x 1mm thick o-ring seals the passage as it crosses the boundary between the crankcase halves. Another o-ring groove was machined around the transfer trough between the gear tower and the crankcase to control oil leakage at their intersection with the block. This groove was designed for a standard 7mm o.d. x 1mm thick o-ring under 10% compression.
A machined passage on the rear surface of the gear tower carries oil from the transfer slot up to the head. After entering a shallow drilled passage in the head, the oil splits into two streams that will carry oil to the cam boxes for eventual distribution to the top-end. A groove for a length of 1mm o-ring cord stock was machined around the vertical passage on the gear tower in order to seal it to the block/head assembly.
The rear face of the front cover had to be slightly modified to clear the pump assembly. A .010" thick Teflon gasket was added to seal it to the crankcase. Finally, mounting holes for the water pump were added to its front side.
An exploded assembly drawing shows the parts behind the front cover related to the oil pumps. Parts for the water pump and starter shaft components will be added later as they are machined. - Terry
The oil manifold holes on the lower front side of the crankcase were trivial in comparison. A .010" thick Teflon gasket was made to seal the manifold to the crankcase.
The tortuous path between the scavenger pump's output and the engine's top-end was completed up through the top of the gear tower. A 2-56 grub screw located in the side of the engine will eventually control its flow. A 3.5mm o.d. x 1mm thick o-ring seals the passage as it crosses the boundary between the crankcase halves. Another o-ring groove was machined around the transfer trough between the gear tower and the crankcase to control oil leakage at their intersection with the block. This groove was designed for a standard 7mm o.d. x 1mm thick o-ring under 10% compression.
A machined passage on the rear surface of the gear tower carries oil from the transfer slot up to the head. After entering a shallow drilled passage in the head, the oil splits into two streams that will carry oil to the cam boxes for eventual distribution to the top-end. A groove for a length of 1mm o-ring cord stock was machined around the vertical passage on the gear tower in order to seal it to the block/head assembly.
The rear face of the front cover had to be slightly modified to clear the pump assembly. A .010" thick Teflon gasket was added to seal it to the crankcase. Finally, mounting holes for the water pump were added to its front side.
An exploded assembly drawing shows the parts behind the front cover related to the oil pumps. Parts for the water pump and starter shaft components will be added later as they are machined. - Terry
Jon James
Member
Jon James
Member
OT.
Hi Terry,
I am building the Hodgson 9 cyl. I have referred to your build thread many times. It has been very helpful to me. I especially appreciate your crankshaft process. It is a much better way to go, with guaranteed accuracy.
Thank you very much for sharing your expertise with all of us.
Of course I am watching your Offy build. Best of luck.
Regards
Jon
Hi Terry,
I am building the Hodgson 9 cyl. I have referred to your build thread many times. It has been very helpful to me. I especially appreciate your crankshaft process. It is a much better way to go, with guaranteed accuracy.
Thank you very much for sharing your expertise with all of us.
Of course I am watching your Offy build. Best of luck.
Regards
Jon
- Joined
- Jun 24, 2010
- Messages
- 2,425
- Reaction score
- 959
Hi Terry, I don't have any deep drilling jobs ahead of me, but I was intrigued by the challenge you had to overcome. What kind of diameter were these holes? What seemed to be the best chip clearing strategy, like short pecks, or drill X amount & complete drill withdrawal? What kind of cutting fluid?
Sidenote, I was having some issues drilling good, straight 0.118" holes in C544 bronze with conventional drills. I switched to a short carbide which went better. Then I tried a parabolic that looks a lot like that Guhring & it went much better despite being HSS & a bit longer length than the carbide. Very different chip removal. Unfortunately, I also decided to try a cream type cutting/tapping 'fluid' that's been kicking around the shop so now I have to separate the two variables to see if the benefit was more one or the other.
Sidenote, I was having some issues drilling good, straight 0.118" holes in C544 bronze with conventional drills. I switched to a short carbide which went better. Then I tried a parabolic that looks a lot like that Guhring & it went much better despite being HSS & a bit longer length than the carbide. Very different chip removal. Unfortunately, I also decided to try a cream type cutting/tapping 'fluid' that's been kicking around the shop so now I have to separate the two variables to see if the benefit was more one or the other.
Peter,
The two deep holes were .093" and .125" in diameter. After getting to half depth I was drilling about .050" at a time and then fully withdrawing. I used WD-40 for the lubricant. The Guhring drills are parabolic. I've found the .093" ones though are too flimsy to start such a deep hole because of so much initial stick-out. I drilled them all manually so I'd 'feel' my way through the process. I was really disappointed that the aircraft drills didn't work, but in hindsight I could have ground down their shanks for some relief. Just glad it's over. An operation that risky is best done early before the part accumulates so much machining time.
The two deep holes were .093" and .125" in diameter. After getting to half depth I was drilling about .050" at a time and then fully withdrawing. I used WD-40 for the lubricant. The Guhring drills are parabolic. I've found the .093" ones though are too flimsy to start such a deep hole because of so much initial stick-out. I drilled them all manually so I'd 'feel' my way through the process. I was really disappointed that the aircraft drills didn't work, but in hindsight I could have ground down their shanks for some relief. Just glad it's over. An operation that risky is best done early before the part accumulates so much machining time.
Just after beginning this build, I received an email from Ron concerning a couple areas in the Offy's design that he felt needs improving. One of these is its cooling system which is currently limiting the engine's runtimes. Ron feels the existing water pump isn't capable of flowing enough coolant through the head's tiny passages.
The current pump is a centrifugal design that's similar to those used in full-size engines. Its impeller, however, is a no-frills straight four blade design with a diameter necessarily limited by the starter shaft that will be located just above it. Ron's suggestion was to replace the pump with a constant displacement design.
Although constant displacement pumps are commonly used with more viscous fluids such as oil, the pumps I've been using to deliver gasoline to the carburetors on my other engines are gear types and have been working well (actually, too well). The pressure and flow requirements in those applications have been practically nil giving me little feel for their performance in a water pump application which will be sensitive to the pump's machining. In the best case, its drive torque could become an issue since it will be driven by the scavenger pump's brass slotted shaft.
Before switching pump types, I decided to compare their performances. My current plan is to construct both of them with outside envelopes similar to Ron's original design. A mocked-up coolant loop similar to the Offy's will be used to make some comparative measurements. Fully completed versions of both pumps should also allow a comparison of their performances later on the actual running engine.
My version of a centrifugal pump is shown in the photos. Its stainless steel impeller is a seven blade seat-of-the-pants design that could most likely be improved by someone who knows what they're doing. Although I increased the blade height from .250" to .312", the impeller's diameter and dual ball bearing supports are identical to those in Ron's pump.
A cover with an integral hose barb was also machined from stainless steel. A flanged tube will connect the pump's output to a similar tube on the block. The pump body itself was machined from 6061 aluminum. It was lapped to the cover and secured with enough 0-80 mounting screws to hopefully avoid the need for a gasket. The bearings are stainless steel and the space between them will be sealed with either graphite string or silicone coated o-rings.
Coolant enters the center of the pump where it picks up kinetic energy from the spinning impeller. The expanded volume near the pump's exit converts some of this energy into a static pressure rise at the pump's outlet. An optimized design within the same space might trade some impeller diameter for a surrounding volute for even higher pressure. Since the energy gained by the fluid should be proportional to the square of the impeller's diameter, I just assumed bigger was better. I didn't know how to account for it, and so cavitation might be a problem.
I hope to have a completed gear pump after the holidays. - Terry
The current pump is a centrifugal design that's similar to those used in full-size engines. Its impeller, however, is a no-frills straight four blade design with a diameter necessarily limited by the starter shaft that will be located just above it. Ron's suggestion was to replace the pump with a constant displacement design.
Although constant displacement pumps are commonly used with more viscous fluids such as oil, the pumps I've been using to deliver gasoline to the carburetors on my other engines are gear types and have been working well (actually, too well). The pressure and flow requirements in those applications have been practically nil giving me little feel for their performance in a water pump application which will be sensitive to the pump's machining. In the best case, its drive torque could become an issue since it will be driven by the scavenger pump's brass slotted shaft.
Before switching pump types, I decided to compare their performances. My current plan is to construct both of them with outside envelopes similar to Ron's original design. A mocked-up coolant loop similar to the Offy's will be used to make some comparative measurements. Fully completed versions of both pumps should also allow a comparison of their performances later on the actual running engine.
My version of a centrifugal pump is shown in the photos. Its stainless steel impeller is a seven blade seat-of-the-pants design that could most likely be improved by someone who knows what they're doing. Although I increased the blade height from .250" to .312", the impeller's diameter and dual ball bearing supports are identical to those in Ron's pump.
A cover with an integral hose barb was also machined from stainless steel. A flanged tube will connect the pump's output to a similar tube on the block. The pump body itself was machined from 6061 aluminum. It was lapped to the cover and secured with enough 0-80 mounting screws to hopefully avoid the need for a gasket. The bearings are stainless steel and the space between them will be sealed with either graphite string or silicone coated o-rings.
Coolant enters the center of the pump where it picks up kinetic energy from the spinning impeller. The expanded volume near the pump's exit converts some of this energy into a static pressure rise at the pump's outlet. An optimized design within the same space might trade some impeller diameter for a surrounding volute for even higher pressure. Since the energy gained by the fluid should be proportional to the square of the impeller's diameter, I just assumed bigger was better. I didn't know how to account for it, and so cavitation might be a problem.
I hope to have a completed gear pump after the holidays. - Terry
That is some impressive work! I'm eager to hear the results.
FWIW - I've seen and heard of gear pumps often in connection with oil, but never water - wonder if the gear pump needs the higher viscosity?
FWIW - I've seen and heard of gear pumps often in connection with oil, but never water - wonder if the gear pump needs the higher viscosity?
I have some knowledge of centrufugal pumps and may be able to confuse you a bit.
First, consider the radial flow through the impeller. You are going from a small diameter to a large one with a constant axial dimension of 5/16". The flow though the inlet has to fill up all that space round the periphery of the impeller, so the radial component of the exit flow velocity is going to be very small. It would be more normal to sweep the blade height to, oh, something like half the pipe bore at the periphery. This is often done by putting a generous bell mouth radius then a slight taper on the inside of the inlet flange.
Second, because of the tight radial clearance, you are preventing any significant flow through the impeller for about two thirds of its periphery. As there is no proper volute, you may find a bit more radial clearance improves matters. Your broad blades are helping you here.
Your outlet arrangement does not help. It would be better if it were tangential. As we see it in the photos: downwards, immediately to the left of the tongue. At the moment you have built up a nice tangential flow velocity and then you just crash it into a wall.
In tiny pumps like this, drag and leakage are significant factors. Make sure the axial clearance between the impeller and the flange is as small as possible (leakage) but putting an outer shroud on the impeller might well not be an improvement because of the increased friction losses.
Without going into the maths, I would say qualitatively that the blade curvature you have is suitable for giving you about as much pressure as you are likely to be able to get. Tiny centrifugal coolant pumps are at a disadvantage in the respect as they will always run at far below their ideal speed. If, on the other hand, it is more flow you need, as may well be the case, then straight vanes, roughly tangential to the eye diameter would be better.
First, consider the radial flow through the impeller. You are going from a small diameter to a large one with a constant axial dimension of 5/16". The flow though the inlet has to fill up all that space round the periphery of the impeller, so the radial component of the exit flow velocity is going to be very small. It would be more normal to sweep the blade height to, oh, something like half the pipe bore at the periphery. This is often done by putting a generous bell mouth radius then a slight taper on the inside of the inlet flange.
Second, because of the tight radial clearance, you are preventing any significant flow through the impeller for about two thirds of its periphery. As there is no proper volute, you may find a bit more radial clearance improves matters. Your broad blades are helping you here.
Your outlet arrangement does not help. It would be better if it were tangential. As we see it in the photos: downwards, immediately to the left of the tongue. At the moment you have built up a nice tangential flow velocity and then you just crash it into a wall.
In tiny pumps like this, drag and leakage are significant factors. Make sure the axial clearance between the impeller and the flange is as small as possible (leakage) but putting an outer shroud on the impeller might well not be an improvement because of the increased friction losses.
Without going into the maths, I would say qualitatively that the blade curvature you have is suitable for giving you about as much pressure as you are likely to be able to get. Tiny centrifugal coolant pumps are at a disadvantage in the respect as they will always run at far below their ideal speed. If, on the other hand, it is more flow you need, as may well be the case, then straight vanes, roughly tangential to the eye diameter would be better.
Another thought. Reading about engine cooling the other day I found that as an estimate you can tke about half the engine power output as being the amount of heat to be rejected by the cooling system. Same source (Judge - Modern Petrol Engines - 1955) suggests a temperature drop across the radiator of 15-20° C. From this you can calculate the necessary flow rate.
Mike Henry
Well-Known Member
- Joined
- Aug 8, 2009
- Messages
- 105
- Reaction score
- 27
We used magnetically coupled gear pumps in an R&D lab to pump water and even ethanol and it seemed to work fine. I believe that the manufacturer specified different gear and seal kits depending on the fluid being pumped.
Good to know - learned something new!
- Joined
- Jul 8, 2009
- Messages
- 760
- Reaction score
- 234
Charles:
You said in Reply #112 - "more flow you need, as may well be the case, then straight vanes, roughly tangential to the eye diameter would be better".
I'm going to ask stupid question, assuming a CW rotation of the impeller, how would the vanes be oriented? In my head I can see two different tangential vane orientations and I'm pretty sure one of them is wrong.
Don
You said in Reply #112 - "more flow you need, as may well be the case, then straight vanes, roughly tangential to the eye diameter would be better".
I'm going to ask stupid question, assuming a CW rotation of the impeller, how would the vanes be oriented? In my head I can see two different tangential vane orientations and I'm pretty sure one of them is wrong.
Don
Before moving on to the gear pump, I decided to test my already completed centrifugal pump. Being anxious to try out some of Charles' suggestions, I also wanted to verify the methods I had used to seal the pump before replicating them in the geared version. My testing was to initially include visibly monitoring the circulation inside a clear plastic loop followed by standalone pressure and flow rate measurements at a couple different pump speeds.
Spinning the pump with a battery powered drill was unwieldy and caused me to spend more time mopping up water than making tests. After cobbling up a test bed around a badly abused Sherline lathe, I was able to spin the pump up to 2000 rpm leaving my hands free to play with the loop. This speed was equivalent to 6000 crankshaft rpm on the Offy.
The Offy will hold some 3 cubic inches of coolant with nearly 90% of it around the sleeves inside the block. A suitably scaled radiator will later add another half cubic inch or so. The dummy coolant loop was made up from a clear plastic bottle and an array of clear plastic tubing to approximate the coolant passages inside the head. The loop's total volume worked out to be just under 4 cubic inches. The flexible tubing allowed me to reconfigure and modify the heights of the various components in the loop and to change the pump's working head pressure.
Although a centrifugal pump isn't self-priming, there's no problem in actual use since it will sit in the lowest part of the coolant loop. Without an overflow tank, some provision should be made to displace air by adding coolant to the loop at it highest point in the engine. In the Offy, a convenient point will be near the end of its water outlet pipe. The array of tiny coolant passages inside the head will tend to trap any air left inside the system, and this can allow the combustion chambers to overheat.
The pump's dismal performance showed up immediately. Below 1000 rpm (3000 crank rpm) there was absolutely no flow through the loop with the simulated block sitting at its proper height above the pump. At 6000 crank rpm there was still no flow until the block was lowered to the same level as the pump. By changing a pulley in my setup I was able to double the rpm and obtain what I would estimate to be a marginally useable flow with the block sitting about an inch above the pump.
I then modified the impeller as suggested by Charles. I radially tapered the height of the blades as shown in one of the photos, and I also reduced the impeller's diameter to allow some circulation around it. The modified impeller did perform better but, below 6000 crank rpm, the pump still could not circulate water around the loop with the block sitting at its required height above the pump.
The flow rate didn't become acceptable until some 12,000 crank rpm when yet another problem showed up. All six connecting passages between the head and block are essentially in parallel. Once circulation began, only one or possibly two of these simulated passages actually flowed coolant leaving much of the 'head' stagnant. Even at 12,000 crank rpm, the centrifugal pump didn't produce enough pressure to promote flow through more than two passages. The diameters of the return tubes in the water outlet pipe on the engine will be graduated to encourage uniform flow across the entire head, but my test bed didn't include this feature.
Based on these tests, I have to conclude that my pretty-looking centrifugal pump would be pretty much non-functional on the Offy. However, I'm not sure its performance is that much different from similar pumps typically used on model engines. Inside a running engine, air expansion and diffusive heat flow through the liquid itself can produce an effective flow giving the impression the pump is doing its job. I remember the first few days of testing my Howell V-4 with its efficient shrouded radiator, overflow tank, and magnetically driven water pump impeller. Even though the expansion tank appeared to be functioning and the radiator was warm, I happen to notice one day that I had forgotten to install the o-ring 'belts' on the water pump pulley. This, along with my experience of the past few days, wouldn't leave me surprised to learn that the centrifugal pumps in many of our model engines may not be doing much of anything.
The bit of good news was that the seals, including the rear silicone coated o-ring seal, appeared to do their jobs and can be carried over to the gear pump. - Terry
Spinning the pump with a battery powered drill was unwieldy and caused me to spend more time mopping up water than making tests. After cobbling up a test bed around a badly abused Sherline lathe, I was able to spin the pump up to 2000 rpm leaving my hands free to play with the loop. This speed was equivalent to 6000 crankshaft rpm on the Offy.
The Offy will hold some 3 cubic inches of coolant with nearly 90% of it around the sleeves inside the block. A suitably scaled radiator will later add another half cubic inch or so. The dummy coolant loop was made up from a clear plastic bottle and an array of clear plastic tubing to approximate the coolant passages inside the head. The loop's total volume worked out to be just under 4 cubic inches. The flexible tubing allowed me to reconfigure and modify the heights of the various components in the loop and to change the pump's working head pressure.
Although a centrifugal pump isn't self-priming, there's no problem in actual use since it will sit in the lowest part of the coolant loop. Without an overflow tank, some provision should be made to displace air by adding coolant to the loop at it highest point in the engine. In the Offy, a convenient point will be near the end of its water outlet pipe. The array of tiny coolant passages inside the head will tend to trap any air left inside the system, and this can allow the combustion chambers to overheat.
The pump's dismal performance showed up immediately. Below 1000 rpm (3000 crank rpm) there was absolutely no flow through the loop with the simulated block sitting at its proper height above the pump. At 6000 crank rpm there was still no flow until the block was lowered to the same level as the pump. By changing a pulley in my setup I was able to double the rpm and obtain what I would estimate to be a marginally useable flow with the block sitting about an inch above the pump.
I then modified the impeller as suggested by Charles. I radially tapered the height of the blades as shown in one of the photos, and I also reduced the impeller's diameter to allow some circulation around it. The modified impeller did perform better but, below 6000 crank rpm, the pump still could not circulate water around the loop with the block sitting at its required height above the pump.
The flow rate didn't become acceptable until some 12,000 crank rpm when yet another problem showed up. All six connecting passages between the head and block are essentially in parallel. Once circulation began, only one or possibly two of these simulated passages actually flowed coolant leaving much of the 'head' stagnant. Even at 12,000 crank rpm, the centrifugal pump didn't produce enough pressure to promote flow through more than two passages. The diameters of the return tubes in the water outlet pipe on the engine will be graduated to encourage uniform flow across the entire head, but my test bed didn't include this feature.
Based on these tests, I have to conclude that my pretty-looking centrifugal pump would be pretty much non-functional on the Offy. However, I'm not sure its performance is that much different from similar pumps typically used on model engines. Inside a running engine, air expansion and diffusive heat flow through the liquid itself can produce an effective flow giving the impression the pump is doing its job. I remember the first few days of testing my Howell V-4 with its efficient shrouded radiator, overflow tank, and magnetically driven water pump impeller. Even though the expansion tank appeared to be functioning and the radiator was warm, I happen to notice one day that I had forgotten to install the o-ring 'belts' on the water pump pulley. This, along with my experience of the past few days, wouldn't leave me surprised to learn that the centrifugal pumps in many of our model engines may not be doing much of anything.
The bit of good news was that the seals, including the rear silicone coated o-ring seal, appeared to do their jobs and can be carried over to the gear pump. - Terry
Last edited:
dnalot
Project of the Month Winner !!!
Hi
Have you considered a vane pump
Have you considered a vane pump
